Shoulder pain is a common medical problem with social and economic consequences: shoulder problems account for more than 12 million visits to physicians annually in the US. A systematic review of literature regarding studies on shoulder pain found that point prevalence ranges from about seven to about 27 percent of the general population of adults less than 70 years of age, with one year prevalence being up to about 47 percent. The wide range is attributed to inconsistent ways in which the shoulder area is defined. Annual incidence rates vary between 0.9 and 2.5% of the general population depending on age. Subacromial impingement syndrome (SIS) is the most common cause of shoulder pain, accounting for 48% of incident cases. Anatomically, SIS refers to the supraspinatus tendon impinging on the undersurface of the anterior acromion as the arm is raised overhead. Typically, pain is generated with elevation of the arm above the head though it can occur with rest. Multiple pathologies, such as subacromial bursitis, rotator cuff tendinopathy, partial rotator cuff tears, and even small tears can coexist to create SIS.
Shoulder pain greatly affects quality of life (QOL). One study found that 84% of subjects with shoulder pain slept less well, 85% had problems moving their arm or hand, and 45% were more irritable. The socioeconomic burden of shoulder pain is also substantial. Shoulder disability can impair one's ability to work and perform household tasks, and results in, on average, 12% lost productive time from work in the US.
Shoulder pain secondary to SIS is not adequately addressed by present therapies. The pain treatment continuum, especially during the acute and subacute phases, begins with conservative treatments such as non-steroidal anti-inflammatory drugs (NSAIDs). Though minimally invasive, these medications are ineffective in the long term for up to half of patients, and commonly have systemic side effects such as headache, skin rash, dizziness, and gastrointestinal symptoms. Other conservative therapies include physical therapy and injections. When ineffective, and as the pain syndrome enters the chronic phase, these conservative therapies are followed by opioid medications or surgical management.
Current treatment options for chronic pain also include physical therapies, oral analgesic medications, local injection techniques, surgery, and neurostimulation. The present treatment options demonstrate marginal pain relief and have undesired side effects. Present neurostimulation methods have clinical and technical difficulties preventing them from becoming the standard of care and more widely adopted. Surface neurostimulation systems are difficult to implement due to the discomfort of stimulation felt on the skin and the need for skilled personnel to place electrodes properly on a daily basis. Implantable neurostimulation systems (e.g., spinal cord stimulation) require placement of the device in the spinal canal (e.g. in the epidural space), which has the potential for nerve damage, unwanted device movement within the spinal column, and repeat clinic visits for re-adjustment. Historically, peripheral nerve stimulators for pain have not achieved widespread clinical success, due to the need to dissect or expose nerves in an open surgical procedure and place leads directly in contact with these target nerves.
Thus, currently available therapies are unsatisfactory in treating shoulder pain. Forty to fifty percent (40-50%) of patients who visit a general practitioner continue to report shoulder pain after 12 months of conservative therapy. Currently there is no commonly accepted standard of care for shoulder pain. Rest (avoiding offending movements such as elevation of the arm over the head), non-steroidal anti-inflammatory drugs, physical therapy, and corticosteroid injections are most commonly used for treating shoulder pain secondary to SIS, regardless of the exact pathology. When these fail, surgery is considered, but surgical pain management due to SIS is no more effective than conservative therapies, leaving 40-50% of patients without an effective treatment for their chronic pain.
Accordingly, the art of shoulder pain therapy would benefit from safe and effective short- and long-term peripheral nerve stimulation (PNS) therapies for patients with moderate to severe acute, sub-acute and even chronic (>6 month) shoulder pain secondary or related to SIS.